Optical image splitting systems

ABSTRACT

In a binocular optical sizing instrument, an adjustable imagesplitting device is arranged, between an objective and the binocular viewing head, for use in measuring the diameters of viewed objects by the image-splitting method. The objective is arranged to form a primary real image of the field of view upstream of the common focal plane of the eyepieces of the binocular viewing head, and optical imaging means are positioned downstream of the primary real image to focus the field of view onto the said common focal plane. The optical system of the instrument is also arranged to form a real image of the objective&#39;&#39;s exit pupil at a point optically equivalent to the apparent image-splitting point of the device. Such an image-splitting device can also be used in a monocular sizing instrument.

united States Patent Smith [451 Mar. 28, 1972 [54] OPTICAL IMAGESPLITTING SYSTEMS 3,326,079 6/1967 Dyson el al. ..356/163 Inventor:Francis Hughes Smith, York, England 3,586,447 6/1971 Vollmer ..356/163[73] Assignee: Vickers Limited, London, England P m y Examiner-Ronaldwibelt Assistant Examiner-J. Rothenberg [22] Med: 1970 Attorney-Pennie,Edmonds, Morton, Taylor and Adams [21] App1.No.: 32,543

[57] ABSTRACT [30] Foreign Application P i it D t In a binocular opticalsizing instrument, an adjustable imagesplitting device is arranged,between an objective and the P 30, 1969 Great Britain binocular viewinghead, for use in measuring the diameters of viewed objects by theimage-splitting method. The objective is [52] US. Cl ..356/l63, 350/33,350/35, arranged to form a primary real image of the field of view up- 3350/173 stream of the common focal plane of the eyepieces of the [51]Int. Cl. ..G01b 11/02 binocular viewing head, and optical imaging meansare posi- Field of Search tioned downstream of the primary real image tofocus the field 3 7 -1 4, 33 of view onto the said common focal plane.The optical system of the instrument is also arranged to form a realimage of the [5 6] References Cited objective's exit pupil at a pointoptically equivalent to the apparent image-splitting point of thedevice. UNITED STATES PATENTS Such an image-splitting device can also beused in a monocu- 1,564,418 12/1925 Konig '.350 45 sizing instrument3,068,743 12/1962 Dyson ...356/l63 3,288,021 11/1966 Hopkins ..356/l6325 Claims, 2 Drawing Figures OPTICAL IMAGE SPLITTING SYSTEMS Thisinvention relates to optical image-splitting systems.

The diameter of a very small object can be measured by means of amicroscope having an image-splitting device between its eyepiece and itsobjective. One such imagesplitting device comprises a collimator and agroup of plane reflectors, constituted by respective surfaces of twoprisms, mounted in an adjustable spatial relationship with one another(one of the prisms being rotatable with respect to the other) upstreamof the eyepiece but downstream of the collimator (by "upstream is meantoptically before, and by downstream" is meant optically after). Whenthis imagesplitting device is in use the group of plane reflectorssplits a primary beam of light, bearing an image of the field of view(including the said object) of the microscope, into two parallelsecondary beams which are separated from one another by a distance whichis selectively variable by adjustment of the said spatial relationship(i.e. by relative rotation of the prisms). The two secondary beams arebrought to a focus in the focal plane of the eyepiece and accordinglytwo intermediate images of the object are formed in the said focalplane. By adjustment of the said spatial relationship, the twointermediate images of the object are caused to move across each otheruntil they are just touching one another edge-to-edge, with no overlap,when a calibrated micrometer scale linked with the adjusted reflectorsurfaces (i.e. with the prisms) indicates the diameter being measured.

The collimator is provided in the aforesaid imagesplitting device inorder to collimate the objectives exit pupil (the exit pupil of anoptical system is an image of the limiting aperture stop of the systemformed by that part of the system which is on the image side of theaperture stop). This is necessary in order to minimize splitting of theexit pupil, by the image- 'splitting device, in a case such as thiswherein the said two secondary beams are parallel to one anotherimmediately downstream of the said group of plane reflectors. Ingeneral, in order to minimize exit pupil splitting, a real image of theexit pupil (of the objective) should be formed at the point from whichthe two secondary beams appear, immediately downstream of the said groupof plane reflectors, to be mutually divergent. The aforesaid casewherein the two secondary beams are parallel is a limiting case in whichthey may be regarded as being mutually divergent from a point atinfinity; accordingly the said collimator is used to provide thecorresponding limiting case in which a real" image of the objectivesexit pupil is formed also at the said point at infinity. Exit pupilsplitting is an effect which can introduce measuring errors and make anaffected instrument difficult to use due to fluctuations of the relativebrightness of the two images seen by an observer when his pupil scansacross the images observed. This is because, near the limits of suchscanning, the observer's pupil becomes cut ofi from overlapping lightbeams, associated respectively with the split exit pupils, successivelyrather than simultaneously, and so the two images observed appear toflicker. If there is no exit pupil splitting, there is no such apparentflickering.

In a binocular microscope it has not proved practicable to insert suchan image-splitting device between a conventional binocular viewing headand the objective that would conventionally be used with that head,because the size of the binocular head, which includes not only the twoeyepieces but also a conventional beam divider therefor, is so greatthat not enough space is left between the head and the objective topermit such insertion. However, it is widely recognized that binocularviewing is less tiring than monocular viewing for prolonged routinemicroscopy, and accordingly it is desirable to provide an opticalimage-splitting system which may be used with binocular viewing.

According to the present invention there is provided an opticalinstrument of the kind having an objective, an eyepeice for viewingintermediate real images, of fields of view under investigation, formedat a focal plane thereof by means of the objective, and animage-splitting device arranged between the objective and the eyepieceand comprising a group of plane reflectors mounted in an adjustablespatial relationship with one another for splitting a primary lightbeam, received from the objective, into two secondary image-bearingbeams which, downstream of the said group, are spaced from one anotherby a distance which is selectively variable by adjustment of the saidspatial relationship, wherein a primary real image of the field of viewis formed by the said objective at a first position downstream of theobjective, first optical imaging means are mounted downstream of theobjective, but upstream of the said group, for forming a real image ofthe objective's exit pupil at a point optically equivalent to a pointfrom which the said two secondary beams appear, immediately downstreamof the said group, to be mutually divergent, and second'optical imagingmeans are mounted downstream of the said group and the said firstposition, but upstream of the said focal plane, for bringing to a focusat the focal plane the images of the field of view that are bornerespectively by the said two secondary beams.

In an instrument embodying the present invention, there are twosuccessive real images, of the field of view, in theobjective-to-eyepiece space, as opposed to the hitherto conventionalone. By this means the optical objective-to-eyepiece distance isincreased to an extent sufficient to permit of the use ofimage-splitting techniques in a binocular microscope.

For a better understanding of the invention, and to show how the samemay be carried into effect, reference will now be made, by way ofexample, to the accompanying drawing in which:

FIG. 1 shows diagrammatically an elevation of the optical system of abinocular microscope embodying the present invention, and

FIG. 2 shows a diagrammatic plan view of a part of the system of FIG. 1.

FIG. 1 shows a binocular microscope comprising a conven tional objective1, an image splitting device 3 and a conventional binocular viewing head4. The objective 1 is shown in a very simplified manner: the objective 1would of course normally be a compound lens. The objective 1 has astandard Royal Microscopical Society mounting thread 2, and the exitpupil of the objective is at 5.

The binocular viewing head 4 comprises the conventional arrangement of abeam divider made up of an interface 7 which transmits and reflects withequal intensities and three reflecting surfaces 8, 9 and 10, and twoeyepieces 11 and 12. The eyepiece 11 has a focal plane 28 and theeyepiece 12 has a focal plane 29. These focal planes are coincident, sothat the eyepieces have a common focal plane 28/29. Like the objectivel, the eyepieces 11 and 12 are shown in a very simplified manner andwould in fact normally be compound lenses.

In summary, when the microscope shown in FIG. 1 is in use, a primarybeam bearing an image of a field of view 30 passes vertically upwardsthrough the objective 1 to an underside of a metallized surface 13, of aprism 14, which reflects the beam into a horizontal looped path in whichthe beam is divided into two secondary beams as described below, thelooped path terminating at the upper side of the surface 13 whereby thetwo secondary beams are discharged vertically into the binocular viewinghead 4.

An effect of the objective on the primary beam is to focus the field ofview at a point along the horizontal looped path thereby to form aprimary orthoscopic image of the field of view in a plane 26 in thevicinity of a field lens 25. The effect of the field lens is to causethe orthoscopic image to converge as it passes through a Porro prism 15,a K-prism 16 and a quarter-wave retardation plate 17 until it reaches afield collimator 18, which collimates the orthoscopic image. The K-prism is a cemented assembly which has a metallized face 19 (FIG. 2) andwhich is mounted so as to be manually rotatable through about the axisXX shown in FIG. 2 between the position shown in FIG. 2 and a positionin which the metallized face 19 occupies a horizontal plane verticallybelow the axis XX.

Having passed through the field collimator 18, the primary beam enters aSwan cube (FIG. 2). At the plane interface of the Swan cube the beam isdivided into the two secondary beams, one reflected and the othertransmitted. The reflected beam (marked with a single arrow) passes fromthe cube through a quarter-wave retardation plate 21 and encounters aplane front surface mirror 22 which reflects the beam back through theplate 21 to the Swan cube interface which then finally transmits thebeam out of the Swan cube. The transmitted beam (marked with a doublearrow) passes from the cube through a quarter-wave retardation plate 23and encounters a plane front surface mirror 24 which reflects the beamback through the plate 23 to the interface which then finally reflectsthe beam out of the Swan cube. The reflecting surfaces of the mirrors 22and 24 lie in vertical planes inclined at right angles to one another.The interface of the Swan cube also lies in a vertical plane. Thus, theplane of the Swan cube interface intersects the planes of the reflectingsurfaces of the mirrors 22 and24 in two lines which are parallel to oneanother. The Swan cube is rotatable manually by micrometercontrolledmounting means (not shown) which may be of conventional type, relativeto the mirrors 22 and 24 about a vertical axis which intersects thehorizontal looped path at a central point of the Swan cube interface.The two mirrors are equidistant from the point of intersection. With theSwan cube in the illustrated orientation, with respect to the mirrors 22and 24, the transmitted beam and the reflected beam recombine at theinterface and there is no image splitting. However, if the Swan cube isrotated through a small angle about its axis of rotation the transmittedbeam and the reflected beam do not recombine at the interface and, onfinally leaving the Swan cube, the two secondary beams are spaced fromone another and appear, immediately downstream of the Swan cube, to bemutually divergent from the reflecting surface of the mirror 22 (bysaying that the two secondary beams are spaced from one another, it ismeant that the central axes of the two beams are spaced from oneanother). The angle by which the two secondary beams diverge from oneanother depends on the angle through which the Swan cube is rotated fromits illustrated orientation.

The secondary beams leaving the Swan cube encounter a verticalreflecting face of the prism 14 whereby the beams are reflected to theupper side of the metallized surface 13, which reflects the beams into avertical plane. The vertically travelling beams pass through a telescopeobjective 6 and a thick quartz plate 27 before entering the binocularviewing head 4. The telescope objective 6 focuses the collimated imagesof the field of view into the common focal plane 28/29 of the twoeyepieces 11 and 1250 that secondary real images are formed in thatplane. These images are viewed through the eyepieces in conventionalmanner, and the image-splitting method of measuring the diameter of anobject in the field of view can accordingly be employed, the aforesaidmicrometer controlled mounting means being appropriately calibrated inconventional manner.

in addition, the exit pupil of the microscope objective 1 serves as anobject for the field lens which converges the image of the'exit pupiltowards a position beyond the mirror 24. The field collimator convergesthe exit pupil further, to bring it to a focus at the reflecting surfaceof the mirror 24. By virtue of the fact that the interface of the Swancube is partly reflecting, the reflecting surface of the mirror 24 is inan optically equivalent position to that of the reflecting surface ofthe mirror 22 and accordingly the exit pupil is really imaged by thelens 25 and the field collimator 18 at a point that is opticallyequivalent to the point from which the two secondary beams appear,immediately downstream of the Swan cube, to be mutually divergent. Byvirtue of the fact that the image of the exit pupil is a point opticallyequivalent to that from which the two secondary beams appear to bemutually divergent there is no exit pupil splitting.

The function of the three quarter-wave retardation plates 17, 21 and 23is to enchance the light transmission and reduce veiling glare clue toair-glass reflections. Oblique incidence at the partially reflectinginterface of the Swan cube 20 gives rise to strong polarization. Forexample the beam which is initially reflected by the interface leavesthe cube with its strongest electrical vibration perpendicular to theplane of incidence on the interface. In order to ensure maximumtransmission when this beam returns through the interface, its outgoingvibration direction is rotated through 90 by virtue of doubletransmission through the quarter-wave plate 20, which is diagonallyoriented. The beam which is initially transmitted by the interfacesleaves the cube with its strongest electrical vibration in the plane ofincidence on the interface and in order to ensure maximum reflectionwhen this beam returns to the interface, its outgoing vibrationdirection is rotated through 90 by virtue of double transmission throughthe quarter-wave plate 23, which is also diagonally oriented. Thepurpose of the plate 17 is to ensure that the secondary beamstransmitted and reflected by the interface have equal amplituderegardless of the state of polarization of the primary beam. The plate17 is also diagonally oriented and so circularly polarizes components ofthe primary beam polarized in and perpendicular to the plane ofincidence at the interface of the cube. The thick quartz plate 27 isrequired because the two secondary beams leave the Swan cube partiallypolarized in mutually perpendicular planes and the beam-splittinginterface 7 exerts a partial analyzing action on the two secondary beamsso that without the plate 27 the two secondary beams would betransmitted and reflected by the interface 7 with different intensities.Such difference of intensities impairs the accuracy of settings made onrotating the Swan cube and the plate 27 is provided to mitigate thiseffect. ideally, the plate 27 circularly polarizes the two secondarybeams so that they are transmitted and reflected by the interface 7 withequal intensities. A further quarter-wave retardation plate may be usedinstead of the thick quartz plate 27.

The image-splitting device. 3 may conveniently be constructed in theform of a module interposed between the oh- 4 jective and the viewinghead of a binocular microscope. The

module could be used with different objectives, each positioned asnearly as is practicable with its exit pupil at the preselectedposition5 in relation to the module 3. A useful degree of avoidance ofexit-pupil splitting may be obtained even if the exit pupil of aparticular objective cannot be placed precisely at the position 5. Thusthe module could usefully be marketed as a separate item, for use inconjunction with binocular viewing heads and objectives alreadyavailable.

Such a module may in fact also be used with a monocular viewing head,but in such a case the thick quartz plate 27 (or the alternative furtherquarter-wave plate) would not be required.

In the device 3 the Swan cube is rotatable about a vertical axis withrespect to the rest of the device. Alternatively, a device could beconstructed in which the Swan cube remains stationary while the twofront surface mirrors are rotated together about a vertical axis. Insuch a case the two component beams again emerge from the Swancube at arelative inclination dependent upon the inclination of the interface ofthe Swan cube with respect to the front surface mirrors;

The provision of the rotable K prism in the illustrated arrangement isnot an essential feature if the microscope has a conventional rotatingstage, but it is a strongly preferred feature because by rotation of theK-prism the secondary real images viewed through the eyepiece can berotated about one another, permitting different diameters of an objectin the field of view to be measured without touching the object itself.

I claim:

1. An optical sizing instrument of the kind comprising an objective anda binocular viewing head having a beam divider and two eyepieces forviewing respective intermediate real images, of a field of view underinvestigation, formed at a common focal plane thereof by means of theobjective, wherein the improvement resides in that:

i. an image-splitting device comprising a group of plane reflectorsmounted in an adjustable spatial relationship with one another isarranged between the objective and the binocular viewing head forsplitting a primary light beam, received from the objective, into twosecondary imagebearing beams which, downstream of the said group, arespaced from one another by a distance which is selectively variable byadjustment of the said spatial relationship;

ii. the objective is arranged to form a primary real image, of the fieldof view, at a first position upstream of the said common focal plane;

iii. first optical imaging means are mounted downstream of theobjective, but upstream of the said group, for forming a real image ofthe objectives exit pupil at a point opticaLly equivalent to a pointfrom which the said two secondary beams appear, immediately downstreamof the said group, to be mutually divergent; and

iv. second optical imaging means are mounted downstream of the saidgroup and the said first position, but upstream of the said common focalplane, for bringing to a focus at the common focal plane the images ofthe field of view that are borne respectively by the said two secondarybeams.

2. An instrument according to claim 1, comprising prism means mountedupstream of the said group for rotation about the primary beam so as torotate the images formed in the said common focal plane from the twosecondary beams.

3. For use in an optical sizing instrument of the kind comprising anobjective and a binocular viewing head having a beam divider and twoeyepieces for viewing respective intermediate real images of a field ofview, formed at a common focal plane thereof by means of the objective,a module adapted to be arranged between the objective and the binocularviewing head and comprising:

i. an image-splitting device comprising a group of plane reflectorsmounted in an adjustable spatial relationship with one another forsplitting a primary light beam, received from the objective, into twosecondary imagebearing beams which, downstream of the said group, arespaced from one another by a distance which is selectively variable byadjustment of the said spatial relationship;

ii. first optical imaging means mounted upstream of the said group, forforming a real image of the objective's exit pupil at a point opticallyequivalent to a point from which the said two secondary beams appear,immediately downstream of the said group, to be mutually divergent; and

iii. second optical imaging means mounted down stream of the said groupfor bringing to a focus at the said common focal plane the images of thefield of view that are borne respectively by the said two secondarybeams.

4. An instrument according to claim 1, wherein the group of planereflectors comprises means presenting a plane partiallyreflectinginterface to the primary beam, and first and second plane reflectorswhich are fixedly positioned relative to one another with their planesintersecting the plane of said partially-reflecting interface inrespective parallel lines and which face towards saidpartially-reflecting interface and which are mounted for rotationrelative to said partially-reflecting interface about an axis that isperpendicular to the primary beam immediately upstream of the said groupand lies in the plane of said interface and is parallel to saidrespective parallel lines.

5. An instrument according to claim 4, wherein the planes of the firstand second reflectors are perpendicular to one another.

6. An instrument according to claim 4, wherein said first and secondplane reflectors comprise respective plane front surface mirrors.

7. An instrument according to claim 4, wherein the planes of the firstand second reflectors are equidistant from the said 3x15.

8. An instrument according to claim 4, wherein the means presenting saidplane partially-reflecting interface to the primary beam comprise a Swancube.

9. An instrument according to claim 1, wherein the two secondary beamsappear, immediately downstream of the said group, to be mutuallydivergent from a point at a finite distance from the second opticalimaging means, and the first optical imaging means comprise a field lensand a field collimator, said field collimator being mounted downstreamof the field lens but upstream of the said group.

10. An instrument according to claim 1, wherein the two secondary beamsappear, immediately downstream of the said group, to be mutuallydivergent from a point at a finite distance from the second opticalimaging means, and the first optical imaging means comprise a field lensand a field collimator mounted downstream of the field lens but upstreamof the said group, the said first position being upstream of the saidgroup and the field lens and the field collimator serving in combinationto collimute the primary real image.

11. An instrument according to claim 10, wherein the imagesplittingdevice, the first optical imaging means and the second optical imagingmeans are parts of an exchangeable module mounted in the instrumentbetween the objective and the binocular viewing head.

12. An instrument according to claim 11, wherein the exchangeable modulefurther includes prism means mounted upstream of the said group, butdownstream of the field lens, for rotation about the primary beam so asto rotate the images formed in the said common focal plane from the twosecondary beams.

13. An instrument according to claim 1, wherein the group of planereflectors comprises means presenting a plane partially-reflectinginterface mounted for splitting said primary beam into a reflected beamand a transmitted beam, and first and second reflection devices, eachhaving at least one plane reflecting surface, arranged for receivingsaid reflected beam and said transmitted beam respectively and returningthem to said interface whereat they are transmitted and reflectedrespectively, the two reflection devices being fixed relative to oneanother and being mounted for rotation relative to saidpartiallyreflecting interface about an axis that is perpendicular to theprimary beam immediately upstream of said group and lies in the plane ofsaid interface.

14. A module according to claim 3, wherein the group of plane reflectorscomprises first and second plane reflectors and means presenting a planepartially-reflecting interface to the primary beam, the first and secondplane reflectors being fixed with respect to one another and facing theplane partiallyreflecting interface, the plane of said interfaceintercepting the planes of the first and second reflectors in respectiveparallel lines, and the means presenting the partiallyreflectinginterface being mounted for rotation with respect to the first andsecond reflectors about an axis that is perpendicular to the primarybeam immediately upstream of the said group and which lies in the planeof the interface parallel to the said parallel lines.

15. A module according to claim 14, wherein the first and second planereflectors constitute respective plane front surface mirrors mounted atright angles to one another.

16. A module according to claim 14, wherein the planes of the first andsecond reflectors are equidistant from the said axis.

17. A module according to claim 14, wherein the means presenting saidplane partially-reflecting interface to the primary beam comprise a Swancube.

18. A module according to claim 3, wherein the two secondary beamsappear, immediately downstream of the said group, to be mutuallydivergent from a point at a finite distance from the second opticalimaging means, and the first optical imaging means comprise a field lensand a field collimator, said field collimator being mounted downstreamof the field lens but upstream of the said group.

19. An optical sizing instrument of the kind comprising an objective andeyepiece means for viewing a field of view under investigation imaged bymeans of the objective at a focal plane of the eyepiece means, whereinthe improvement resides in that:

i. an image-splitting device comprising a group of plane reflectorsmounted in an adjustable spatial relationship with one another isarranged between the objective and the eyepiece means for splitting aprimary light beam, received from the objective, into two secondaryimagebearing beams which, downstream of the said group, are spaced fromone another by a distance which is selectively variable by adjustment ofthe said spatial relationship;

ii. the objective is arranged to form a primary real image, of the fieldof view, at a first position upstream of the said focal plane;

iii. first optical imaging means are mounted downstream of theobjective, but upstream of the said group, for forming a real image ofthe objectives exit pupil at a point optically equivalent to a pointfrom which the said two secondary beams appear, immediately downstreamof the said group, to be mutually divergent; and

iv. second optical imaging means are mounted downstream of the saidgroup and the said first position, but upstream of the said focal plane,for bringing to a focus at the said focal plane the images of the fieldof view that are borne respectively by the said two secondary beams.

20. An instrument according to claim 19, comprising prism means mountedupstream of the said group for rotation about the primary beam so as torotate the images formed in the said focal plane from the two secondarybeams.

21. An instrument according to claim 19, wherein the group of planereflectors comprises means presenting a plane partiallyreflectinginterface to the primary beam, and first and second plane reflectorswhich are fixedly positioned relative to one another with their planesintersecting the plane of said partially-reflecting interface inrespective parallel lines and which face towards saidpartially-reflecting interface and which are mounted for rotationrelative to said partiallyreflecting interface about an axis that isperpendicular to the primary beam immediately upstream of the said groupand lies in the plane of said interface and is parallel to saidrespective parallel lines.

22. An instrument according to claim 21, wherein the first and secondreflectors comprise respective plane front surface mirrors mounted atright angles to one another.

23. For use in an optical sizing instrument of the kind comprising anobjective and eyepiece means for viewing a field of view imaged by meansof the objective at a focal plane of the eyepiece means, a moduleadapted to be arranged between the objective and the eyepiece means andcomprising:

i. an image-splitting device comprising a group of plane reflectorsmounted in an adjustable spatial relationship with one another forsplitting a primary light beam, received from the objective, into twosecondary imagebearing beams which, downstream of the said group, arespaced from one another by a distance which is selectively variable byadjustment of the said spatial relationship;

. first optical imaging means mounted upstream of the said group, forforming a real image of the objectives exit pupil at a point opticallyequivalent to a point from which the said two secondary beams appear,immediately downstream of the said group, to be mutually divergent; and

iii. second optical imaging means mounted downstream of the said groupfor bringing to a focus at the said focal plane the images of the fieldof view that are borne respectively by the said two secondary beams.

24. A module according to claim 23, wherein the group of planereflectors comprises means presenting a plane partiallyreflectinginterface to the primary beam, and first and second plane reflectorswhich are fixedly positioned relative to one another with their planesintersecting the lane of saidartially-reflecting interface in respectiveparal el lines and w ich face towards said partially-reflectinginterface and which are mounted for rotation relative to saidpartially-reflecting interface about an axis that is perpendicular tothe primary beam immediately upstream of the said group and lies in theplane of said interface and is parallel to said respective parallellines.

25. A module according to claim 24, wherein the first and secondreflectors comprise respective plane front surface mirrors fixed atright angles to one another.

1. An optical sizing instrument of the kind comprising an objective anda binocular viewing head having a beam divider and two eyepieces forviewing respective intermediate real images, of a field of view underinvestigation, formed at a common focal plane thereof by means of theobjective, wherein the improvement resides in that: i. animage-splitting device comprising a group of plane reflectors mounted inan adjustable spatial relationship with one another is arranged betweenthe objective and the binocular viewing head for splitting a primarylight beam, received from the objective, into two secondaryimage-bearing beams which, downstream of the said group, are spaced fromone another by a distance which is selectively variable by adjustment ofthe said spatial relationship; ii. the objective is arranged to form aprimary real image, of the field of view, at a first position upstreamof the said common focal plane; iii. first optical imaging means aremounted downstream of the objective, but upstream of the said group, forforming a real image of the objective''s exit pupil at a point opticallyequivalent to a point from which the said two secondary beams appear,immediately downstream of the said group, to be mutually divergent; andiv. second optical imaging means are mounted downstream of the saidgroup and the said first position, but upstream of the said common focalplane, for bringing to a focus at the common focal plane the images ofthe field of view that are borne respectively by the said two secondarybeams.
 2. An instrument according to claim 1, comprising prism meansmounted upstream of the said group for rotation about the primary beamso as to rotate the images formed in the said common focal plane fromthe two secondary beams.
 3. For use in an optical sizing instrument ofthe Kind comprising an objective and a binocular viewing head having abeam divider and two eyepieces for viewing respective intermediate realimages of a field of view, formed at a common focal plane thereof bymeans of the objective, a module adapted to be arranged between theobjective and the binocular viewing head and comprising: i. animage-splitting device comprising a group of plane reflectors mounted inan adjustable spatial relationship with one another for splitting aprimary light beam, received from the objective, into two secondaryimage-bearing beams which, downstream of the said group, are spaced fromone another by a distance which is selectively variable by adjustment ofthe said spatial relationship; ii. first optical imaging means mountedupstream of the said group, for forming a real image of the objective''sexit pupil at a point optically equivalent to a point from which thesaid two secondary beams appear, immediately downstream of the saidgroup, to be mutually divergent; and iii. second optical imaging meansmounted downstream of the said group for bringing to a focus at the saidcommon focal plane the images of the field of view that are bornerespectively by the said two secondary beams.
 4. An instrument accordingto claim 1, wherein the group of plane reflectors comprises meanspresenting a plane partially-reflecting interface to the primary beam,and first and second plane reflectors which are fixedly positionedrelative to one another with their planes intersecting the plane of saidpartially-reflecting interface in respective parallel lines and whichface towards said partially-reflecting interface and which are mountedfor rotation relative to said partially-reflecting interface about anaxis that is perpendicular to the primary beam immediately upstream ofthe said group and lies in the plane of said interface and is parallelto said respective parallel lines.
 5. An instrument according to claim4, wherein the planes of the first and second reflectors areperpendicular to one another.
 6. An instrument according to claim 4,wherein said first and second plane reflectors comprise respective planefront surface mirrors.
 7. An instrument according to claim 4, whereinthe planes of the first and second reflectors are equidistant from thesaid axis.
 8. An instrument according to claim 4, wherein the meanspresenting said plane partially-reflecting interface to the primary beamcomprise a Swan cube.
 9. An instrument according to claim 1, wherein thetwo secondary beams appear, immediately downstream of the said group, tobe mutually divergent from a point at a finite distance from the secondoptical imaging means, and the first optical imaging means comprise afield lens and a field collimator, said field collimator being mounteddownstream of the field lens but upstream of the said group.
 10. Aninstrument according to claim 1, wherein the two secondary beams appear,immediately downstream of the said group, to be mutually divergent froma point at a finite distance from the second optical imaging means, andthe first optical imaging means comprise a field lens and a fieldcollimator mounted downstream of the field lens but upstream of the saidgroup, the said first position being upstream of the said group and thefield lens and the field collimator serving in combination to collimatethe primary real image.
 11. An instrument according to claim 10, whereinthe image-splitting device, the first optical imaging means and thesecond optical imaging means are parts of an exchangeable module mountedin the instrument between the objective and the binocular viewing head.12. An instrument according to claim 11, wherein the exchangeable modulefurther includes prism means mounted upstream of the said group, butdownstream of the field lens, for rotation about the primary beam so asto rotate the images formed in the said common focal plane from the twosecondary beams.
 13. An instrument according to clAim 1, wherein thegroup of plane reflectors comprises means presenting a planepartially-reflecting interface mounted for splitting said primary beaminto a reflected beam and a transmitted beam, and first and secondreflection devices, each having at least one plane reflecting surface,arranged for receiving said reflected beam and said transmitted beamrespectively and returning them to said interface whereat they aretransmitted and reflected respectively, the two reflection devices beingfixed relative to one another and being mounted for rotation relative tosaid partially-reflecting interface about an axis that is perpendicularto the primary beam immediately upstream of said group and lies in theplane of said interface.
 14. A module according to claim 3, wherein thegroup of plane reflectors comprises first and second plane reflectorsand means presenting a plane partially-reflecting interface to theprimary beam, the first and second plane reflectors being fixed withrespect to one another and facing the plane partially-reflectinginterface, the plane of said interface intercepting the planes of thefirst and second reflectors in respective parallel lines, and the meanspresenting the partially-reflecting interface being mounted for rotationwith respect to the first and second reflectors about an axis that isperpendicular to the primary beam immediately upstream of the said groupand which lies in the plane of the interface parallel to the saidparallel lines.
 15. A module according to claim 14, wherein the firstand second plane reflectors constitute respective plane front surfacemirrors mounted at right angles to one another.
 16. A module accordingto claim 14, wherein the planes of the first and second reflectors areequidistant from the said axis,
 17. A module according to claim 14,wherein the means presenting said plane partially-reflecting interfaceto the primary beam comprise a Swan cube.
 18. A module according toclaim 3, wherein the two secondary beams appear, immediately downstreamof the said group, to be mutually divergent from a point at a finitedistance from the second optical imaging means, and the first opticalimaging means comprise a field lens and a field collimator, said fieldcollimator being mounted downstream of the field lens but upstream ofthe said group.
 19. An optical sizing instrument of the kind comprisingan objective and eyepiece means for viewing a field of view underinvestigation imaged by means of the objective at a focal plane of theeyepiece means, wherein the improvement resides in that: i. animage-splitting device comprising a group of plane reflectors mounted inan adjustable spatial relationship with one another is arranged betweenthe objective and the eyepiece means for splitting a primary light beam,received from the objective, into two secondary image-bearing beamswhich, downstream of the said group, are spaced from one another by adistance which is selectively variable by adjustment of the said spatialrelationship; ii. the objective is arranged to form a primary realimage, of the field of view, at a first position upstream of the saidfocal plane; iii. first optical imaging means are mounted downstream ofthe objective, but upstream of the said group, for forming a real imageof the objective''s exit pupil at a point optically equivalent to apoint from which the said two secondary beams appear, immediatelydownstream of the said group, to be mutually divergent; and iv. secondoptical imaging means are mounted downstream of the said group and thesaid first position, but upstream of the said focal plane, for bringingto a focus at the said focal plane the images of the field of view thatare borne respectively by the said two secondary beams.
 20. Aninstrument according to claim 19, comprising prism means mountedupstream of the said group for rotation about the primary beam so as torotate the images formed in the said focal plane from the two secondarybeams.
 21. An instrument according to claim 19, wherein the group ofplane reflectors comprises means presenting a plane partially-reflectinginterface to the primary beam, and first and second plane reflectorswhich are fixedly positioned relative to one another with their planesintersecting the plane of said partially-reflecting interface inrespective parallel lines and which face towards saidpartially-reflecting interface and which are mounted for rotationrelative to said partially-reflecting interface about an axis that isperpendicular to the primary beam immediately upstream of the said groupand lies in the plane of said interface and is parallel to saidrespective parallel lines.
 22. An instrument according to claim 21,wherein the first and second reflectors comprise respective plane frontsurface mirrors mounted at right angles to one another.
 23. For use inan optical sizing instrument of the kind comprising an objective andeyepiece means for viewing a field of view imaged by means of theobjective at a focal plane of the eyepiece means, a module adapted to bearranged between the objective and the eyepiece means and comprising: i.an image-splitting device comprising a group of plane reflectors mountedin an adjustable spatial relationship with one another for splitting aprimary light beam, received from the objective, into two secondaryimage-bearing beams which, downstream of the said group, are spaced fromone another by a distance which is selectively variable by adjustment ofthe said spatial relationship; ii. first optical imaging means mountedupstream of the said group, for forming a real image of the objective''sexit pupil at a point optically equivalent to a point from which thesaid two secondary beams appear, immediately downstream of the saidgroup, to be mutually divergent; and iii. second optical imaging meansmounted downstream of the said group for bringing to a focus at the saidfocal plane the images of the field of view that are borne respectivelyby the said two secondary beams.
 24. A module according to claim 23,wherein the group of plane reflectors comprises means presenting a planepartially-reflecting interface to the primary beam, and first and secondplane reflectors which are fixedly positioned relative to one anotherwith their planes intersecting the plane of said-partially-reflectinginterface in respective parallel lines and which face towards saidpartially-reflecting interface and which are mounted for rotationrelative to said partially-reflecting interface about an axis that isperpendicular to the primary beam immediately upstream of the said groupand lies in the plane of said interface and is parallel to saidrespective parallel lines.
 25. A module according to claim 24, whereinthe first and second reflectors comprise respective plane front surfacemirrors fixed at right angles to one another.